Nuclear power I

8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further fission reactions (chain reactions).

When Uranium-235 undergoes a decay, neutrons are produced. These neutrons get trapped by other Uranium-235 atoms, which will then undergo a decay as well, and produce other neutrons and so on. This is called chain reaction, because one Uranium decay will make three other Uranium atoms to decay. (Holt Physics, 2006)

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When the neutron has a low energy (about 1eV) it is more likely to be absorbed by the uranium atom, so it will favour nuclear fission.

The critical mass is the minimum of fuel that is "sufficiently large for a self-sustaining chain reaction to take place." (Giancoli, 1998)

8.4.2 Distinguish between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).

"A nuclear power station involves controlled nuclear fission whereas an uncontrolled nuclear fission produces the huge amount of energy released in nuclear weapons. Weapons have been designed using both uranium and plutonium as the fuel." (Tim Kirk, 2007, p 69)

Nuclear weapons have some moral and ethical issues. It leads for example to a Cold War between the US and the Soviet Union. The nuclear weapons that can kill thousands of people such as the ones in Hiroshima and Nagasaki are banned since the Geneva convention because of their ethical issues.

8.4.3 Describe what is meant by fuel enrichment.

"Naturally occurring uranium contains less than 1% of uranium-235. Enrichment is the process by which this percentage composition is increased to make nuclear fission more likely." (Tim Kirk, 2007, p 69)

8.4.4 Describe the main energy transformations that take place in a nuclear power station.

"A nuclear power station involves controlled nuclear fission. [...] The fuel used, hydrogen, is in plentiful supply and the reaction (if it could be sustained) would not produce significant amounts of radioactive waste. [...] [It] requires creating temperatures high enough to ionize atomic hydrogen into a plasma state (this is the 'fourth state of matter', in which electrons and protons are not bound in atoms but move independently)." (Tim Kirk, 2007, p 69)

Watch this video for an easy understanding of the processes.

8.4.5 Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.

Moderator: "Collisions between the neutrons and the nuclei of the moderator slow them down and allow further reactions to take place." (Tim Kirk, 2007, p 69)

Control rods "are movable roads that readily absorb neutrons. They can be introduced or removed from the reaction chamber in order to control the chain reaction." (Tim Kirk, 2007, p 69)

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8.4.6 Discuss the role of the heat exchanger in a fission reactor.

"The heat exchanger allows the nuclear reactions to occur in a place that is sealed off from the rest of the environment. The reactions increase the temperature in the core. This thermal energy is transferred to water and the steam that is produced turns the turbines." (Tim Kirk, 2007, p 69)
It is represented by the purple chamber on the diagram above.